Effects of Interaction between Humic Acid and Birnessite on Antimony Speciation in Yellow Soil from Antimony Mining Areas

Antimony (Sb) contamination in mining area soils is severe, and its environmental risk mainly depends on chemical speciation of Sb. Mn oxides and humic acids are key active components regulating Sb speciation in soil. At present, most of the research focuses on the single adsorption behavior toward Sb, but the interaction between them is less, especially for real soil environment. Using an in situ preparation method, 0.1% birnessite (δ-MnO₂) was loaded on the Sb-heavily contaminated yellow soil in the mining area. Humic acid with 1.0%, 2.0%, 3.0%, and 5.0% of the soil mass was added. After 40 days of flooding, the changes of various forms of Mn and Sb in the soil were analyzed, and the influence mechanism of the interaction between humic acid and δ-MnO₂ on the transformation of various forms of Sb in the soil was explored. The results indicate that with increase of humic acid addition, the content of suspended Sb and weakly acid-extractable Sb in the original soil group decrease by 20.7 time and 75.8%, respectively. Meanwhile, the content of reducible Sb, oxidizable Sb, and residual Sb increase by 75.4%, 41.0%, and 4.6%, respectively. In the Mn-loaded soil group, suspended Sb and weakly acid-extractable Sb decrease by 41.2 times and 84.6 times, respectively, while reducible Sb, oxidizable Sb, and residue Sb increase by 83.2%, 2.9 times, and 7.9 times, respectively. Compared with the original soil, the contents of weakly acid-extractable Sb and reducible Sb in Mn-loaded soil increase by 19.4%–36.7% and 41.3%-–77.6%, respectively. When humic acid content is greater than 3.0%, the content of oxidizable Sb increases by 26.5%–38.2%. Humic acid and δ-MnO₂ can synergistically convert highly mobile suspended Sb and weakly acid-extractable Sb in soil into more stable reducible, oxidizable, and residual forms, thereby reducing Sb migration. These findings provide a critical theoretical support and practical pathway for the synergistic remediation of Sb-contaminated soils using Mn oxides and humic acid.